Effects of phosphoric acid concentration and etch duration on enamel depth of etch: an in vitro study.

In a previous study we reported no significant differences among the shear bond strengths resulting from the application of an orthodontic bonding resin to enamel surfaces etched with three phosphoric acid (H3PO4) concentrations, each for three etch durations. The objective of the current study was to determine the depths of etch on ground enamel surfaces exposed to the nine etching procedures. The facial surfaces of 45 extracted human maxillary permanent central incisors were ground wet on 600-grit silicon carbide paper. Annular adhesive disks of 6 mm outer diameter and 3 mm inner diameter were positioned on the ground enamel surfaces and etched with 10 mm3 of 37%, 15%, and 5% H3PO4 for 60, 30, and 15 seconds, respectively. The calcium concentrations of the etching solutions were determined and the depths of etch calculated. The depths of etch were then measured with a surface profilometer. A stepwise decrease in the calculated depths of etch with decreasing acid concentration and duration of etching was obtained. The calculated etch depths ranged from 27.1 microns by etching with 37% H3PO4 for 60 seconds to 3.5 microns by etching with 5% H3PO4 for 15 seconds. The measured depths of etch followed a similar pattern. A highly significant correlation between calculated and measured depths of etch was obtained.

Effects of phosphoric acid concentration and etch duration on the shear bond strength of an orthodontic bonding resin to enamel. An in vitro study.

The purpose of this in vitro study was to determine the effects of phosphoric acid (H3PO4) concentration and duration of etching on the shear bond strength of an orthodontic bonding resin to enamel. Nine bonding procedures, each involving 18 extracted human maxillary permanent canines, were used. Ground enamel surfaces were etched with a 37% H3PO4 solution, a 15% H3PO4 gel, or a 5% H3PO4 solution for 60, 30, and 15 seconds, respectively. Cylinders of an orthodontic bonding resin, Concise, were prepared in a special device. The test specimens were disassembled 15 minutes after preparation and stored in distilled water at 37 degrees C for 24 hours. A shear load was applied to the bonded cylinders at a crosshead speed of 0.02 in.min-1 in an Instron testing machine, and the shear bond strengths were calculated and expressed in MN.m-2. A two-factor analysis of the data showed that the H3PO4 concentration had no significant effect on the shear bond strength, but the duration of etching affected shear bond strength significantly. The enamel aspects of the fractured test specimens were examined microscopically and the percent failure within the bonding resin at the bonding sites estimated. The correlation between shear bond strength and percentage failure within the bonding resin was not significant. The effects of the nine etching procedures on ground and unground enamel surfaces were studied by scanning electron microscopy. The etching procedures produced well-defined etching patterns on both ground and unground enamel surfaces.